1. Field of the Invention
The present invention relates to a composite material having high thermal conductivity, which is used as a heatsink material for semiconductor devices, and a method for producing the same.
2. Description of the Prior Art
Generally speaking, in order to radiate heat generated from incorporated semiconductor elements, a heatsink made of a high thermal conductivity material is attached to a semiconductor device. The standard of physical properties required for a heatsink material is such that the thermal conductivity is equivalent to or more than that of Cu (395 W/mxc2x7K) and the coefficient of thermal expansion is lower than that of Cu (16.9xc3x9710xe2x88x926/xc2x0C.).
Conventionally, Al2O3 and AlN whose coefficient of thermal expansion is approximate to that of a semiconductor element have been used as a heatsink material in spite of their comparatively low thermal conductivity because conventional electronic components incorporating semiconductor elements such as semiconductor lasers, microwave elements, etc., have generated only a slight amount of heat.
Recently, however, in line with an increase in the amount of information, semiconductor elements have been increased in size, and the output thereof has been highly increased. As a result, an increase in the amount of heat generation causes a problem. For Example, AlN has consistently been used till recently because it has comparatively satisfactory thermal conductivity as well as the coefficient of thermal expansion similar to that of Si and InP, but AlN can no longer meet further increase in the output and size of semiconductor elements in view of its thermal conductivity. Therefore, recently, in order to incorporate these semiconductor elements having high output, heatsink materials having remarkably excellent thermal conductivity are demanded.
Further, in view of the coefficient of thermal expansion, AlN is not suitable as a heatsink material for semiconductor elements that are composed of material, such as GaAs, which has a large coefficient of thermal expansion. In detail, the coefficient of thermal expansion of various types of semiconductor materials is expressed in terms of xc3x9710xe2x88x926/xc2x0C. (hereinafter expressed in terms of ppm/xc2x0C.), wherein Si is 4.2, InP is 4.5, and GaAs is 5.9 or so. Therefore, it is recommended that the coefficient of thermal expansion of heatsink materials is close to these figures. In addition, a heatsink material preferably should have a low Young""s modulus so that the generation of thermal stress is reduced.
Although the material having the highest thermal conductivity is diamond and c-BN (cubic boron nitride), their coefficient of thermal expansion is very low, wherein that of diamond is 2.3 ppm/xc2x0C. and that of c-BN is 3.7 ppm/xc2x0C., and the Young""s modulus of these materials is very high to be 830 through 1050 GPa. Therefore, a large thermal stress occurs when brazing a heatsink and a semiconductor element together or between the heatsink and semiconductor element when being used as a device, wherein a breakage is likely to occur.
Recently, various types of composite materials such as Alxe2x80x94SiC, in which ceramic and metal are composed together, have been proposed as a heatsink material having a low coefficient of thermal expansion as well as comparatively high thermal conductivity. However, since the thermal conductivity of Al is as low as approx. 238 W/mxc2x7K at room temperature, there exists an upper limit in the thermal conductivity of a composite material including Al, which results in the failure to meet the recent requirements for high thermal conductivity such as described above. The composite material cannot meet recent requirements in order to achieve the high thermal conductivity as described above. It may be considered that metals having high thermal conductivity such as Cu (395 W/mxc2x7K at room temperature) and Ag (420 W/mxc2x7K at room temperature) are used instead of Al. However, since the wettability thereof with SiC is very inferior, the high thermal conductivity that is inherent in Cu and Ag cannot be sufficiently displayed.
Japanese Unexamined Patent Publication No. 11-67991 discloses a diamond-Ag based or diamond-Cu based composite material as a heatsink material having improved wettability with Cu and Ag. According to the disclosed invention, a diamond powder and Agxe2x80x94Cuxe2x80x94Ti based powder are blended together and molded, and then are heated at a higher temperature than the melting point of the resultant alloy. This allows Ti constituents to diffuse on the surface of diamond grains and to react to form a TiC film on the surface (sintering method). Since the TiC has good wettability with Cu or melted Ag, the phase boundaries of the diamond grains and the metal are adhered close to each other, whereby high thermal conductivity can be obtained.
Also, an infiltration method is disclosed in Japanese Unexamined Patent Publication No. 10-223812 as a method for producing such a diamond-Ag based or diamond-Cu based composite material. In this method, after diamond powder and Agxe2x80x94Cuxe2x80x94Ti based powder are blended and molded, the molded body is heated at a higher temperature than the melting point of the corresponding alloy to form a TiC layer on the surface of diamond grains. After that, the molded body is further heated to elute and volatilize the Ag constituents and Cu constituents, thereby producing a porous body. Impregnating the porous body with an Agxe2x80x94Cu alloy produces a composite material having a higher relative density and a higher thermal conductivity than that obtained by the sintering method.
However, there are common problems in the case of the above-described diamond-Ag based and diamond-Cu based composite materials: (1) the diamond is remarkably expensive, (2) the diamond has very high hardness, which allows a large thermal stress to remain at the phase boundary of bonding between the composite material and semiconductor element due to a high Young""s modulus as described above, and (3) metal molding dies are remarkably worn when a blended powder including diamond is molded. Resultantly, the cost of a diamond-Ag based and diamond-Cu based composite material becomes very high, and it is very difficult to employ the same in practical applications. Also, (4) even if the infiltration method is employed, a problem still remains, it is difficult to make the diamond-Ag based and diamond-Cu based composite materials completely dense.
The present invention was developed in views of these situations, and it is therefore an object of the invention to provide, without the use of expensive diamond, a composite material that is favorable when it is used as a heatsink material while it is low cost, has a low coefficient of thermal expansion and has comparatively high thermal conductivity.
In order to achieve the above-described object, the invention provides a high thermal conductivity composite material consisting of a first constituent composed of composite carbon grains, composite carbon fibers, or composite carbide grains, which have a coating layer formed on the surface thereof, and a second constituent composed of a metal including silver and/or copper; wherein the coating layer formed on the surface of the composite carbon grains, composite carbon fibers, or composite carbide grains, which are the first constituent, is composed of carbide of at least a type of metal selected from the group consisting of 4A group elements, 5A group elements, and 6A group elements of the periodic table. The high thermal conductivity composite material has a relative density of 70% or more, thermal conductivity of 220 W/mxc2x7K or more at least in a specified direction at room temperature, and a mean coefficient of thermal expansion of 5 through 15xc3x9710xe2x88x926/xc2x0C. from room temperature to 200xc2x0 C. at least in a specified direction.
In the above-described high thermal conductivity composite material according to the invention, where the first constituent is made of composite carbon grains having a coating layer formed on the surface thereof or composite carbon fiber having the same, it is preferable that the ratio of content of the carbon grains or carbon fibers is 30 through 95% by volume fraction. Also, where the above-described first constituent is composite carbide grains having a coating layer formed on the surface thereof, it is preferable that the ratio of content of the carbide grains is 15 through 85% by volume fraction.
In the above-described high thermal conductivity composite material according to the invention, it is preferable that the coating layer formed on the surface of the above-described first constituent is titanium carbide, and the thickness of the coating layer formed on the surface of the first constituent is 0.01 through 3 xcexcm, preferably 0.05 through 1 xcexcm. And, where the second constituent is made of silver or copper, it is preferable that the ratio of content of copper in the second constituent is 20% by volume fraction or less or 80% by volume fraction or more. Since the thermal conductivity of an Agxe2x80x94Cu alloy is lowered unless the above-described alloy composition is obtained, there is a tendency that the thermal conductivity of the composite material is lowered.
In the high thermal conductivity composite material according to the invention, preferably, the relative density is 95% or more, and the thermal conductivity is 250 W/mxc2x7K or more at least in a specified direction at room temperature. More preferably, the relative density is 99% or more, and the thermal conductivity is 270 W/mxc2x7K or more at least in a specified direction at room temperature.
Such a high thermal conductivity composite material according to the invention is preferably used as a semiconductor heatsink member. In addition, the present invention provides a semiconductor apparatus using a semiconductor heatsink member made of the above-described high thermal conductivity composite material.
The present invention provides a method for producing a high thermal conductivity composite material comprising a first constituent composed of composite carbon grains, composite carbon fibers, or composite carbide grains, which have a coating layer formed on the surface thereof, and a second constituent composed of a metal including silver and/or copper. That is, the method for producing a composite material based on a sintering method according to the invention comprises the first step of preparing graphite powder, carbon fibers, or carbide powder, and simultaneously preparing alloy powder including at least a type of metals selected from the elements belonging to 4A, 5A and 6A groups, whose main constituent is silver and/or copper, of the periodic table; the second step of molding a mixture of the powder and making the same into a molded body; and the third step (3a) of heating the molded body in a vacuum state whose pressure is 0.0133 Pa or less, or in a gas atmosphere containing helium, argon or hydrogen at a higher temperature than the melting point of the alloy, forming a coating layer consisting of at least a type of metal carbides selected from the elements belonging to 4A, 5A and 6A groups of the periodic table on the surface of graphite grains, carbon fibers or carbide grains; and simultaneously making the same into a sintered body.
Further, the invention provides a method for producing the above-described high thermal conductivity composite material based on the infiltration method. That is, the producing method on the basis of the infiltration method comprises the first step of preparing graphite powder, carbon fibers, or carbide powder, and simultaneously preparing alloy lumps or alloy powder or a molded body thereof including at least a type of metals selected from the elements belonging to 4A, 5A and 6A groups, whose main constituent is silver and/or copper, of the periodic table; the second step of molding the graphite powder, carbon fibers, or carbide powder and making the same into a molded body; and the third step (3b) of bringing the molded body into contact with the alloy lumps or alloy powder or their molded body, heating the same at a higher temperature than the melting point of the alloy in a vacuum state whose pressure is 0.0133 Pa or less, or in a gas atmosphere including helium, argon or hydrogen, forming a coating layer composed of at least a type of metal carbides, which is selected from the elements belonging to 4A, 5A and 6A groups of the periodic table, on the surface of graphite grains, carbon fibers, or carbide grains, and simultaneously making the same into a sintered body, and infiltrating the melted alloy into the sintered body.
In the method for producing a high thermal conductivity composite material according to the invention, wherein, after a part of metal is eluted or volatilized by increasing the sintering temperature or lengthening the sintering time in the third step 3a or 3b to make the same into a porous body, the porous body is brought into contact with metal lumps or metallic powder of silver and/or copper or their molded body, and is heated at a higher temperature than the melting point of the metal in a non-oxidized atmosphere, and the melted metal is infiltrated into the porous body to make the same into an infiltrated body. Also, the sintered body that is obtained in the third a or b step may be preheated at a higher temperature than the melting point of the metal in the sintered body in the atmosphere or an inert gas atmosphere, and may be forged with a pressure equivalent to 600 MPa or more.
In the above-described method for producing a high thermal conductivity composite material according to the invention, the heating temperature in the third step exceeds the melting point of the alloy powder and is (the melting point plus 50)xc2x0C. or less. Also, in each of the second steps of the respective producing methods, it is preferable that the molded body is obtained through a hydrostatic pressure molding process in a cold or hot state. At this time, it is preferable that the molding pressure is 300 MPa or more.
Still further, the invention provides a method for producing a high thermal conductivity composite material composed of a first constituent consisting of composite silicon carbide grains having a coating layer formed on the surface thereof, and a second constituent consisting of a metal consisting of silver and/or copper. That is, the method comprises the steps of: pressurizing and sintering silicon carbide powder at a higher temperature than 2000xc2x0 C. in an inert gas atmosphere or a vacuum state, and forming a porous body of silicon carbide; and bringing the porous body of silicon carbide into contact with an alloy lump or alloy powder, whose main constituent is silver and/or copper, including at least a type of metals selected from the elements belonging to 4A, 5A and 6A groups of the periodic table, or its molded body, heating the porous body at a higher temperature than the melting point of the alloy in a vacuum state whose pressure is 0.0133 Pa or less or a gas atmosphere of helium, argon or hydrogen, forming a coating layer consisting of at least a type of metal carbides selected from the elements belonging to 4A, 5A and 6A groups of the periodic table on the surface of silicon carbide grains, and simultaneously infiltrating the melted alloy into the porous body.
In the above-described method for producing the high thermal conductivity composite material, after the obtained composite material is heated at a higher temperature than the infiltrating temperature and a part of the metal is eluted and volatilized to make the same into a porous body again, the porous body is brought into contact with a metal lump of silver and/or copper, or metallic powder thereof, or its molded body, and is heated to a higher temperature than the melting point of the metal in a non-oxidized atmosphere, and the melted metal is infiltrated into the porous body.
Therefore, the present invention is able to provide a composite material having comparatively high thermal conductivity with the coefficient of thermal expansion limited, without the use of any expensive diamond. Also, with the composite materials according to the invention, it is possible to produce heatsink members whose thermal conductivity is high as in diamond and whose coefficient of thermal expansion is very close to that of semiconductor elements. By using the heatsink members, it is possible to display fill performance of semiconductor lasers, microwave devices, and various types of LSI.